1. Field of the invention
This invention relates to a method of coding a picture signal to increase average run length and apparatus therefor.
2. Description of the prior art
Run length coding viz., the coding of lengths of consecutive black and white picture elements is well known in the art of facsimile transmission for the purpose of transmission redundancy reduction. This technique allows for the compression of a two-level picture obtained by scanning a black and white drawing or document.
The Modified Huffman code and the two dimensional extension of the same, called the Modified READ code, are examples of international standards established for the purposes of increasing the utility of the above mentioned method.
However, even though run length coding has found extensive use in redundancy reduction in two-level facsimile digital transmissions, the same efficiency is not expected with the image processing of a half-tone picture. More specifically, a half-tone picture is reproduced, through dithering or screening, by controlling spatial density of black and white picture elements, and hence the number of runs increases due to reduced run lengths. Thus, the compression efficiency of picture data decreases to a considerable extent.
Furthermore, a picture with continuous gray level can not be run length coded using the internationally standardized algorithms of run length coding. Image signals of such a picture are usually coded by Differential PCM technique, for example.